Calcining the ore is an exothermal process. Usually cooling bundles are disposed in the stationary fluidized bed in order to cool the fluidized bed and maintain it at the desired temperature. The transverse mixing of the solid material is relatively weakly developed in the stationary fluidized bed; it is impeded further by the cooling bundles. This leads to the fluidized bed becoming hotter at the point of introduction of the ore than at points where no ore is supplied. Moreover, much sulfur dioxide is obtained at the point of introduction of the ore due to the reaction of the sulfide sulfur with the atmospheric oxygen, whereas at points where little or no ore is supplied, little sulfur dioxide can be obtained, so that only a small part of the atmospheric oxygen supplied is consumed for calcining. In the furnace space above the stationary fluidized bed, there are mostly temperatures above 900° C., and the gas velocities are below 1 m/sec.
Because of the high viscosity of the gases, mixing of the gas streaks rich in sulfur dioxide and of the gas streaks poor in sulfur dioxide hardly occurs. Therefore, pronounced inhomogeneities with respect to temperature, sulfur dioxide concentration and oxygen concentration occur in the furnace space. In gas streaks which are poor in sulfur dioxide but rich in oxygen, an undesired sulfatizing of the solid material preferentially occurs. This leads to the deterioration of the product. Furthermore, sulfatized dust tends to form deposits on the tubes of following waste heat boilers, which deposits can hardly be removed. The transport of heat from the calcining gas to the boiler tube is impeded thereby, the costs for maintaining the boiler are increased, and its availability is reduced.